Multicast DNS service discovery, aka. Zeroconf or Bonjour, is a useful means of making your node app (e.g. multiplayer game or IoT project) easily discoverable to clients on the same local network.
The node_mdns module worked out-of-the-box on my Mac. Unfortunately things weren’t as straightforward on a node-alpine docker container running on Raspberry Pi Zero, evidenced by this error at runtime:
Error: dns service error: unknown
at new Browser (/home/app/node_modules/mdns/lib/browser.js:86:10)
at Object.create [as createBrowser] (/home/app/node_modules/mdns/lib/browser.js:114:10)
Here’s how I managed to solve this. The following was pieced together from a number of sources (linked at the end).
I’ll assume you have a node app using node_mdns to publish your service, and a Dockerfile based on alpine-linux to build your app into an image for running on the Pi.
Firstly, you’ll need to have the alpine packages to run the avahi daemon, along with its development headers and compat support for bonjour. I.e. in your Dockerfile:
FROM arm32v6/node:10-alpine3.9 # Avahi is for DNS-SD broadcasting on the local network; DBUS is how Avahi communicates with clients RUN apk add python make gcc libc-dev g++ linux-headers dbus avahi avahi-dev avahi-compat-libdns_sd
You’ll need to make sure the DBus and Avahi daemons are started in your container before starting your node app. Since you can only execute a single startup command from your Dockerfile, we’ll need to bundle the commands into a startup script, and run that. In your Dockerfile:
ENTRYPOINT ["./startup.sh"]
And startup.sh:
#!/usr/bin/env sh dbus-daemon --system avahi-daemon --no-chroot & node index.js # your app script here
Note: --no-chroot is added to avoid this runtime error:
alpine linux netlink.c: send(): Not supported
Build your Docker image (since this is for a Pi Zero in my case, I’m using DockerX to build for the ARMv6 architecture on my Mac. I recommend this over waiting days or weeks for it to build on the Pi Zero):
docker buildx build -t myapp --platform linux/arm/v6 -o type=docker .
Now push then pull your Docker image onto your Raspberry Pi. If you don’t want to use a cloud-hosted registry, I’d recommend taking a look into setting up a local registry to push it directly to the Pi on your local network.
To run your docker image on the Pi, you’ll first need to disable the host OS’s avahi-daemon (if any) to prevent conflicts with the avahi-daemon that will be running inside your alpine-linux container. On Raspbian, you can disable avahi with:
# SSH into your Pi sudo systemctl disable avahi-daemon
Then to run your docker image:
docker run -d --net=host localhost:5000/myapp
(localhost:5000 here refers to a local docker registry.) Using the host’s network (--net=host) seems to be necessary for mDNS advertisements to function. In theory you should be able to just map port 5353/udp from the container, but this didn’t work. (If you happen to know why, please drop a comment below).
That’s it. If all goes well you should be able to see your service advertised on the local network. E.g. from a Mac on the same network (the last line is our node app’s http service):
$ dns-sd -B _services._dns-sd._udp Browsing for _services._dns-sd._udp DATE: ---Fri 29 May 2020--- 22:05:29.580 ...STARTING... Timestamp A/R Flags if Domain Service Type Instance Name 22:05:29.582 Add 3 10 . _tcp.local. _hue 22:05:29.582 Add 3 10 . _tcp.local. _hap 22:05:29.582 Add 3 10 . _tcp.local. _workstation 22:05:29.582 Add 3 10 . _tcp.local. _ssh 22:05:29.582 Add 3 10 . _tcp.local. _sftp-ssh 22:05:29.582 Add 2 10 . _tcp.local. _http
Full sample source code on github: https://github.com/kiwiandroiddev/node-alpine-docker-mdns
Credits/References
https://hub.docker.com/r/stanback/alpine-avahi
https://github.com/homebridge/homebridge/issues/613
https://github.com/joyent/smartos-live/issues/669
https://github.com/home-assistant/docker/issues/23
https://stackoverflow.com/questions/30646943/how-to-avahi-browse-from-a-docker-container